In the UMTS (Universal Mobile Telecommunications System) network, the specifications of long-term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1).
In LTE, as multiple-access schemes, a scheme that is based on OFDMA (Orthogonal Frequency Division Multiple Access) is used in downlink channels (downlink), and a scheme that is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used in uplink channels (uplink).
Also, successor systems of LTE—referred to as, for example, “LTE-advanced” or “LTE enhancement”—have been studied for the purpose of achieving further broadbandization and increased speed beyond LTE, and the specifications thereof have been drafted as LTE Rel. 10/11 (LTE-A).
As duplex modes for radio communication in LTE systems and LTE-A systems, there are frequency division duplex (FDD) to divide between the uplink (UL) and the downlink (DL) based on frequency, and time division duplex (TDD) to divide between the uplink and the downlink based on time.
Also, the system band of LTE Rel. 10/11 includes at least one component carrier (CC), where the LTE system band constitutes one unit. Such bundling of a plurality of CCs into a wide band is referred to as “carrier aggregation” (CA).
In carrier aggregation (CA) in LTE Rel. 10/11, the duplex modes to employ in a plurality of CCs have to be the same duplex mode. By contrast with this, for example, in carrier aggregation (CA) of future radio communication systems including LTE Rel. 12 and later versions, a plurality of CCs might employ varying duplex modes. Carrier aggregation (CA) of this type will be referred to as “TDD-FDD CA.”
In LTE Rel. 12, which is a more advanced successor system of LTE, various scenarios to use a plurality of cells in different frequency bands (carriers) are under study. When the radio base stations to form a plurality of cells are substantially the same, the above-described carrier aggregation (CA) is applicable. When the radio base stations to form a plurality of cells are completely different, dual connectivity (DC) may be employed.
Carrier aggregation (CA) may be referred to as “intra-eNB CA,” and dual connectivity may be referred to as “inter-eNB CA.”